1. Field of the Invention
The present invention relates to a projection optical system and projection apparatus using the same and, particularly, is suitably applicable, for example, to liquid crystal projectors with a fixed shift mechanism for magnifying and projecting image information based on a liquid crystal element, onto a screen surface located above the projection optical axis.
2. Related Background Art
The liquid crystal projectors (projection apparatus) for projecting image information based on a liquid crystal display device (liquid crystal panel, LCD), onto a predetermined surface have widely been used heretofore for conferences, presentations, etc., for example, as apparatus capable of presenting a projected image from a personal computer or the like on a large screen.
Under such circumstances the projection optical system for the liquid crystal projectors is demanded to achieve a short distance from the screen to the apparatus and to project the magnified, projected image to a possibly highest position, i.e., upward. One of methods for upward projection is a method of intentionally shifting and fixing the center of the liquid crystal display device from the optical axis of the projection optical system (so called a shift projection optical system), thereby magnifying and projecting the image, mainly, to the upper side of the optical (reference) axis of the projection optical system as to the magnification-side conjugate (screen) surface.
Namely, as shown in FIG. 14, concerning the demagnification-side conjugate surface, the liquid crystal display device is shifted and fixedly positioned so that the center of the liquid crystal display device LCD is located mainly on the lower side of the optical (reference) axis La of the projection optical system PL. For this reason, there are many cases where only an approximately half region (on the lower side) of an original, effective image circle (effective field) of the projection optical system PL, which is indicated by hatching in the drawing, is used.
FIG. 14 is a view from the R channel out of R (red), G (green), and B (blue) channels.
In general, in the case of large-scale liquid crystal projectors and the like for implementing superimposed projection of two or more projected images on a screen through the use of a plurality of liquid crystal projectors like two or three projectors, there are cases where a shift amount of the liquid crystal display device is variable. However, the shift mechanism of the liquid crystal display device is used in a fixed state, particularly, in the compact and lightweight liquid crystal projectors using the 0.9-inch or smaller liquid crystal display device, or rather, the fixed shift mechanism is adopted for compact and lightweight structure, in order to avoid enlargement and complexity of the set with the variable shift mechanism.
Besides, another means for upward projection is a projection method of obliquely tilting the projection optical system relative to the screen.
On that occasion trapezoid distortion and image inclination occurs due to an angle of inclination between the screen surface and the optical (reference) axis of the projection system and a projection apparatus correcting them with an optical system is proposed, for example, in Japanese Patent Application Laid-Open No. 9-304732.
In recent years, with increase in the number of pixels of the liquid crystal display device or the like, there are desires for a bright, large-aperture projection optical system that has high resolution and that can be used even under bright fluorescent lamps and under daylight. On the other hand, there are strong desires for compact and lightweight projectors of 5 pounds or less using the compact liquid crystal display device.
For meeting the desires, high-resolution and compact zoom lenses consisting of four or five lens units are recently proposed as projection lenses (projection optics) for the liquid crystal projectors, for example, in Japanese Patent Applications Laid-Open No. 10-186235, Laid-Open No. 10-268193, and so on.
The trapezoid distortion appears in the projected image in the case where the image display element (liquid crystal display device) is used with a shift relative to the optical axis of the projection optical system or in the case where the projection optical system is used with a tilt of the optical axis relative to the screen. Even in the case where the optical axis of the projection apparatus is not tilted relative to the screen, the center of the image display element disagrees with the optical axis and thus the projection optical system is used in an asymmetric manner with respect to the optical axis, which causes the image to be projected in a distorted state, for example, if distortion aberration remains in the projection optical system.
Japanese Patent Application Laid-Open No. 9-304732 discloses the means for optically correcting the trapezoid distortion and image inclination in the upward projection in the liquid crystal projector, but it is difficult to apply the means to a vari-focal (zoom) optical system as it is, because it raises complexity in shapes of lenses and lens surfaces constituting the projection optical system and forces considerable loads on processing.
In general, for making the projection optical system brighter and higher in resolution, it is necessary to properly set the lens configuration of each lens, in order to search for a configuration enabling high resolution while maintaining a large full-open pupil diameter.
Without proper setting of the lens configuration of each lens unit, for example, though the entire optical system can be constructed in compact scale, movement of the lens units will become complex during the zoom operation and the entire projection apparatus will become complex, for example, because of increase in the number of lens barrels.